Nozzles which act as fluid amplifiers have been available in the past, some of which operate by using the Coanda effect. The Coanda effect is the tendency of a gas or liquid coming out of a jet to travel close to the wall contour even if the wall's direction of curvature is away from the jet's axis. One nozzle that uses the Coanda effect has been disclosed in U.S. Pat. No. 3,806,039 (by Mocarski), in which the nozzle has a through passageway in which the cross-sectional opening at the inlet is quite large, then tapers down to a much narrower cross-sectional area at the throat of the nozzle.
In the Mocarski nozzle, ambient fluid is introduced into the inlet at the large cross-sectional area. A pressurized fluid is introduced from the side of the nozzle into an annular passageway. The fluid is further directed into a slot which contains a washer or shim. The washer or shim has rectangular spokes that are directed inward, but terminate before reaching the center portion of the shim to allow an open area through which ambient fluid can pass through the nozzle without any restriction from the spoked washer. As the pressurized fluid enters the slot area, some of the fluid travels along the surface of the spoked washer forming a jet at the center portion of the washer where the spokes terminate. At that point, the pressurized fluid flow gives up velocity to induce mass flow of ambient fluid. This pulls fluid through the inlet at the large cross sectional area and it mixes with the pressurized air to exit at the discharge end.
The Mocarski nozzle controls its slot width precisely by the spoked washer's thickness. In addition, by controlling the ratio of blocked area to open area (where the washer has spokes or no spokes), the open area of the slotted washer is precisely controlled, which is important for controlling the pressurized fluid flow. Unfortunately, an air amplifier nozzle constructed according to Mocarski operates effectively at some air pressures and washer thicknesses, but at other air pressures and other washer thicknesses it is ineffective. In some circumstances, the pressurized air flow can blow out both ends (both the inlet and discharge ends) of the nozzle, or the pressurized air flow will not follow the Coanda profile to provide the proper performance as an air amplifier. In addition, at low compressed air supply pressures, the Mocarski nozzle has very little ability to work against a back pressure, which makes it ineffective for use in most air amplifier applications. At high compressed air supply pressures, the Mocarski nozzle can have its pressurized air flow blow out both ends and have little ability to work against a back pressure. It has been determined that the high velocity air flow through a nozzle constructed according to the Mocarski patent does not follow the Coanda profile because of entrainment of ambient air in areas of the nozzle where the ambient air flow is blocked, while no compressed air flow is available to overcome that problem.